At the second glance this issue doesn't seem as simple. If passwall goes through unknown squares on its way to exclusion then it indeed yields some extra information to the player and should take some time therefore. Should prompt behaviour differ for these situations?

Also imagine the following situation:

@#X#.

# is rock wall, X - excluded unknown square. If excluded square is a rock wall, passwall will not be interrupted by the prompt. If this square is empty, the prompt will be issued. Is this wrong?